Scaled plots of electromagnetic data

ABSTRACT

Method for improving frequency-domain ( 1505 ), controlled-source electromagnetic data readability and interpretability in hydrocarbon prospecting by mapping a scaled amplitude or the relative amplitude (or phase) of the electromagnetic field in a scaled offset−scaled frequency plane ( 1510 ). The preferred mapping space uses the offset times the square-root of the frequency as the X-axis and the square-root of the frequency as the Y-axis. The preferred way to scale the amplitude of the electric field is to multiply it by the frequency to the power of −1.5. Resistive anomalies in the data may be identified by comparing negative offset data (or data relative to a reference) to positive offset data ( 1515 ). The location and size of a potentially hydrocarbon-bearing resistive body may be estimated from the position of the anomaly on the map ( 1520 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage entry under 35 U.S.C. 371 ofPCT/US2008/004342 that published as WO 2008/156517 and was filed on Apr.3, 2008 and claims the benefit of U.S. Provisional application60/934,519 which was filed on 14 Jun. 2007.

FIELD OF THE INVENTION

The invention relates generally to the field of geophysical prospecting,and more particularly to electromagnetic surveying. Specifically, theinvention is a method of mapping controlled source electromagneticsurvey data to enhance identification of resistive anomalies thatpotentially are caused by hydrocarbon formations.

BACKGROUND OF THE INVENTION

Since about the year 2000, Controlled Source ElectroMagnetism (“CSEM”)has grown from the research stage to commercial applications for oil andgas marine exploration (Constable & Srnka, Geophysics 72, WA3-WA12(March-April 2007)). Typically, in offshore CSEM, a vessel tows asubmerged CSEM source (also called transmitter) above the sea floor.Electric and/or magnetic fields are recorded at receivers lying on thesea floor. Conventionally, negative offsets correspond to the distancesbetween the transmitter locations and the receiver when the transmitteris approaching the receiver. Positive offsets correspond to thedistances between the receiver and the transmitter locations when thetransmitter is moving away from the receiver. Processing transforms therecorded time series of the electric and magnetic fields into frequencydomain data at discrete frequencies. The high frequencies in the sourcesignal are attenuated in their journey from source to receiver morequickly than the low frequencies. Thus, the high frequencies do notpenetrate as deep as the low frequencies, but, in the other hand, theresolution of the low frequency data is poorer. The decay of theelectric or magnetic fields with offset is controlled by the resistivityof the subsurface. A conductive earth induces a more rapid decay.Resistivity anomalies like oil or gas accumulations produce anomalouslyslower decays. CSEM interpretation consists in detecting and makingsense of those anomalies.

With increasing experience around the world, it has appeared that theearth resistivity structure is more complicated than initiallyanticipated. More data are required to reduce the interpretationambiguity. Nowadays, it is common to use denser surveys (more lines andmore receivers) and several frequencies to improve lateral and verticalresolution. The technical problem addressed by the present invention ishow to overcome the daunting problem of displaying a huge amount of datain a meaningful way.

It is difficult to detect the presence of an anomaly from a first glanceat conventional parametric plots such as FIGS. 1A-B, which show thesurvey data recorded at one receiver from one line of towed transmitter.The X-axis (i.e. the horizontal axis) corresponds to the distancebetween the receiver and the transmitter locations (offsets). Thelocation of the receiver corresponds to the X=0 line. The Y-axis (i.e.the vertical axis) of FIG. 1A corresponds to the amplitude of theelectric field (note the logarithmic scale) and the Y-axis of FIG. 1Bcorresponds to the phase of the electric fields. Magnetic field datawould yield similar pictures. The parameter in FIGS. 1A-B is frequency.(Several frequencies can be extracted from typical survey data.) Datafor six frequencies ranging from 0.125 Hz (101) to 2 Hz (102) are shown(arrow 103 indicating the direction of increasing frequency), but manymore frequencies can be extracted from waveforms used in modern surveys.In the case of FIGS. 1A-B, there is a known, small anomaly that extendsto the right (positive offsets) of the receiver, 600 meters below thesea floor. At a first glance it is difficult to tell that the rightsides of the plots look more resistive, or any different at all, thanthe left sides. Plotting several frequencies only increases theconfusion on the plot.

Since the data interpreter is looking for anomalies, it is convenient toestimate or simulate the normal, i.e. background, electro-magneticresponse and to compare the recorded data to this reference. FIGS. 2A-Cshow the variation of the electric field at four receivers (the tiplocations of the inverted “V's” correspond to receiver locations, alsoindicated by reference number 205) at 3 different frequencies (0.125 Hzfor FIG. 2A, 0.25 Hz for FIG. 2B and 2 Hz for FIG. 2C) along one towline. The darker points correspond to the amplitude of the recordedfields and the lighter-shade lines to the simulation of a referenceearth, without anomaly (background simulation). From the high frequencydata (2 Hz, FIG. 2C), it is obvious that the measured data are above thesimulated curves on the right side of the plot (201). This indicates aresistivity anomaly. A close examination of the lower frequency plots(FIGS. 2A-B) also show a more resistive character on the right sides ofthe plots (202) and (203), but the response is much less obvious (lackof resolution). This way of displaying data is obviously not verysatisfactory and can be very confusing if many receivers and frequenciesare considered.

The picture can be simplified by considering only the ratio between theobserved data and the reference (S. Ellingsrud, et al., The Leading Edge21, 972-982, (2002)). FIGS. 3A-C show the ratio of the actual dataamplitude to the reference amplitude for the same four receivers (1, 2,3 and 4) at three frequencies (the same frequencies as in FIGS. 2A-C)along a tow line. The receiver reference number is also used to indicatethe amplitude ratio curve corresponding to that receiver. Left ofreceiver 4 (region 301), the ratio is very close to 1, i.e. there is noanomaly. Right of receiver 4 the maximum ratio varies from greater than2 at 2 Hz (best resolution, region 302) to smaller than 1.25 at 0.125 Hz(lowest resolution, region 303). It may be noted that the high frequencydata do not extend as far as the low frequency data because they aremore quickly attenuated and rapidly fall below the noise level. The edgeof the resistivity anomaly is located right below receiver 4 (theanomaly extends to the right), but the ratio is not significantlygreater than one at the location of receiver 4. Accurately picking theedge of the anomaly is difficult. Three different plots are required toshow the information at the three different frequencies. Plotting alldata together on one figure would make a very confusing picture.

Other publications dealing with improved ways of displaying orinterpreting CSEM data include US Patent Publication US/2006/0197534 andPCT International Publication WO 2006/096328.

SUMMARY OF THE INVENTION

In one embodiment, the invention is a hydrocarbon prospecting method fordisplaying electromagnetic field data from a controlled sourceelectromagnetic survey of a subsurface region to enhance identificationof resistive anomalies in the subsurface region, thereby locatingresistive bodies causing the anomalies, comprising:

-   -   (a) decomposing electromagnetic data from at least one survey        receiver into frequency domain, and selecting amplitude or phase        data corresponding to at least two frequencies (step 1505 in the        flow chart of FIG. 15);    -   (b) plotting a quantity representing the selected        electromagnetic field data represented on the plot by contours        or color scale on a coordinate plane where the axes are        corresponding scaled frequency and scaled offset        (source-receiver separation), with scale factors being chosen to        produce substantially vertical (parallel to frequency axis)        contours in offset ranges not impacted by resistive anomalies        (step 1510);    -   (c) identifying one or more resistive anomalies from the plot,        for example by comparing positive offset data to negative offset        data (step 1515); and    -   (d) estimating location of a resistive body and potential        hydrocarbon source that is causing an anomaly from location of        the anomaly on the plot relative to location of the survey        receivers (step 1520).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its advantages will be better understood byreferring to the following detailed description and the attacheddrawings in which:

FIGS. 1A-B are conventional parametric plots of amplitude and phase datafrom a single receiver, traditionally used for interpreting CSEM data;

FIGS. 2A-C are conventional plots at three source frequencies displayingdata from four different receivers along a source tow line on the sameplot;

FIGS. 3A-C are amplitude ratio plots at three different frequencies;

FIG. 4B is a plot of phase vs. scaled offset for multiple frequenciesaccording to Willen's method; FIG. 4A is a plot of scaled amplitude vs.scaled offset for multiple frequencies according to the presentinvention;

FIG. 5 is an amplitude contour map in offset-frequency space;

FIG. 6 is a scaled amplitude contour map in scaled offset/scaledfrequency space;

FIG. 7 is a contour map displaying an attribute (gradient of scaledamplitude) in the scaled offset/scaled frequency plane;

FIG. 8 is a map in which scaled amplitude and gradient of scaledamplitude are co-rendered in the scaled offset/scaled frequency plane;

FIG. 9 is the map of FIG. 8, after muting out noise;

FIG. 10 is a map of relative amplitude in the scaled offset/scaledfrequency plane;

FIG. 11 is a map of relative phase in the scaled offset/scaled frequencyplane;

FIG. 12 is a map of relative amplitude in offset/frequency space;

FIG. 13 is a multi-receiver map of relative amplitude in scaledoffset/scaled frequency space;

FIG. 14 illustrates calculation of the electric distance between areceiver and a resistive body causing and anomaly in the survey data;and

FIG. 15 is a flow chart showing basic steps in one embodiment of thepresent inventive method.

The invention will be described in connection with its preferredembodiments. However, to the extent that the following detaileddescription is specific to a particular embodiment or a particular useof the invention, this is intended to be illustrative only, and is notto be construed as limiting the scope of the invention. On the contrary,it is intended to cover all alternatives, modifications and equivalentsthat may be included within the scope of the invention, as defined bythe appended claims.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention is a method for improving CSEM data readabilityand interpretability by mapping a scaled amplitude or the relativeamplitude (or phase) of the electromagnetic field in a scaledoffset−scaled frequency plane. The preferred mapping space uses theoffset times the square-root of the frequency as the X-axis and thesquare-root of the frequency as the Y-axis. The preferred way to scalethe amplitude of the electric field is to multiply it by the frequencyto the power of −1.5.

As shown on FIG. 1B, the curves of the phase of electromagnetic fieldversus offset are very different at different frequencies, even if theyare recorded over a homogenous medium (negative offsets in FIG. 1B). Itis not easy to detect the difference in the phase curves that is inducedby a resistivity anomaly. It is disclosed in PCT InternationalPublication No. WO2007/092070 that instead of plotting phase vs. offsetas is typically done, if the same phase data is plotted versus theoffset multiplied by the square root of the frequency, then the familyof parametric curves such as in FIG. 1B collapse to a single curve 401in a homogenous medium as shown in FIG. 4B, and that the resistivityanomaly that was virtually undetectable in FIG. 1B causes a prominentdifference in the curves at 402.

A discovery of the present invention is that a similar collapsing effectis observed in the amplitude vs. scaled offset plots if the amplitude ismultiplied by the frequency raised to power −1.5 as indicated bycollapsed curve 403 in FIG. 4A. If a resistive anomaly is present, as itis in the data used for FIGS. 1A-B and 4A-B, it produces a smallereffect 404 for the amplitude curves than for the phase curves (402).Nevertheless, the anomaly can be detected in FIG. 4A whereas it would bevery difficult to detect in FIG. 1A.

In FIG. 5, the amplitude of the electromagnetic field of FIG. 1A iscontoured in the Offset−Frequency plane. Amplitude is the parameter inthis parametric plot instead of frequency. A similar picture would beobserved for the phase. Constant offset lines are shown by verticalbroken lines 501. Thick, black lines such as 502 parallel to the offsetaxis (X-axis) indicate the six frequencies where actual data arepresent, from 0.125 Hz (503) to 2 Hz (504). An obvious advantage of FIG.5 compared to FIG. 1A is that many frequencies can be added to thepicture without obscuring it. However any difference between the leftside (negative offsets) of the picture (no anomaly) and the right side(resistive anomaly is present) remains unclear. On both sides of thezero-offset line, the contour lines of the amplitude (or the phase) arecurved. The contours represent amplitude values varying from 10⁻¹⁵V/A·m²at the large offsets to 10⁻⁸V/A·m² at offsets close to zero.

However, if the same amplitude data are multiplied by the frequencyraised to the negative 1.5 power and are contoured in the Scaled Offset(Offset×square-root of frequency)−Scaled Frequency (Square root offrequency) plane, the difference between both sides becomes obvious, asshown on FIG. 6. In this plane, constant offset lines are displayed asthin, oblique broken lines; e.g., 605 corresponds to an offset of +5 km,606 to −5 km, 607 to +3 km, 608 to +1 km, etc. On the left side of thepicture (no anomaly), the contours 601 of the scaled amplitude(Amplitude×Frequency^(−1.5)) are straight lines, almost parallel to theY-axis. On the right side of the picture, the contours 602 are bentbetween the offset lines 608 and 607 corresponding to 1 and 3 km-offset.The presence of the resistive anomaly is now clearly indicated.

In this Scaled Offset−Scaled Frequency plane, it is possible to computemap attributes, like gradient, azimuth, first derivative, secondderivative etc. to highlight subtle variations in the phase andamplitude of the data. FIG. 7 shows the gradient of the scaledamplitude, represented as the parameter in the parametric family ofplots (same data as in FIGS. 1A-B, 4A-B, 5 and 6). On the left side (noanomaly), the close vertical contours 701 for the short scaled-offsetscorrespond to the initial steep curves of amplitude. Then, for furtherscaled-offsets 702, the gradient contours are more spaced but they arestill almost vertical, i.e. almost parallel to the Y-axis. On the rightside the same initial steep gradient is observed for shortscaled-offsets 703 until the resistivity anomaly effect is felt (704).

Of course, data displays such a FIG. 7 can and typically will beco-rendered with attribute information represented in a continuous wayby a color scale instead of parametrically by contour lines. This isquite helpful in detecting data problems. In FIG. 8, the contours of thescaled amplitude are co-rendered with the color-coded (gray scalesubstituted for patent purposes) gradient (data come from anotherreceiver). There is some noise in polygons 801 and 802. Those polygonscan be interactively drawn on the picture and the noisy data can bemuted out. FIG. 9 shows the figure after muting. Interruptions withinovals 901 and 902 of the data at the discrete frequencies 502 show wherethe data were muted. The contour lines are interpolated across themissing data. Such a scaled plot display enables the user to manipulatedata from different frequencies at the same time.

Although the display of FIG. 6 represents a considerable improvementover previous methods for anomaly detection and interpretation, anotherembodiment of the present inventive method is typically even better.That embodiment is a display of the ratio between the amplitude of theobserved data to a reference amplitude (or the difference between thephase of the data and the phase of the reference) in the ScaledOffset−Scaled Frequency plane, as illustrated in FIG. 10. FIG. 10 isbased on the same electric field data as in FIGS. 2 and 3 plus datacorresponding to three additional frequencies. On the left side of thedisplay (negative offsets), the logarithm of the amplitude ratio (alsocalled relative amplitude) is very close to zero, as indicated by thegray scale. The observed data are very close to the reference data,indicating that there is no resistivity anomaly. On the right side, theanomaly is obvious, even without comparison to the left side, or even ifboth sides looked the same. Thus, resistive anomalies can be identifiedfrom the shape of the contours or from color differences when comparingpositive offset data to negative offset data on a plot such as FIG. 10or, more generally, when comparing different receiver locations on theplot. Note how well the colors (gray shades) and contours are organizedacross scaled offset and scaled frequency in FIG. 10. Such anorganization was not clear in FIGS. 2 and 3.

The relative amplitude contours 1006 are fairly parallel in the vicinityof the 1 km-constant-offset oblique line 1002 (referred to as 608 inFIG. 7). Even if the exact location of the zero-contour is not easy topick because of noise, the direction of these contours is a very robustpiece of information. They are aligned with the 1.2 km constant offsetline 1003 (thick black broken line). This distance of 1.2 km is hereincalled the electric distance between the receiver and the anomaly. Inthis case, the anomaly edge is directly below the receiver, 600 m belowthe sea floor. The electric distance is the geometric (straight line)distance between the receiver and the anomaly plus the distance betweenthe anomaly and the transmitter (approximately the depth of theanomaly).

This relationship can be inferred from plots such as FIG. 10. It can beshown that the first contours have the same 1.2 km-slope for all thereceivers above the anomaly (which is at a constant depth of 0.6 km inthis example). If the anomaly were at a depth of 2 km, the firstcontours in the scaled-offset scaled-frequency plane would align alongthe 4 km direction (i.e., the 4 km constant offset line) for all thereceivers above the anomaly. Thus, if the receivers are above theanomaly the contours are aligned to the twice-depth-of-the-anomalydirection. FIG. 13 will show that if the receivers are no longerdirectly above the anomaly, the first contours are aligned with the H+Zdirection, where H is the closest distance between the receiver and theanomaly and Z the closest distance between the transmitter and theanomaly (roughly the depth of the anomaly (below the sea floor) becausethe transmitters are close to the sea floor. This relationship of thepresent invention seems to hold true as long as the shape of the anomalyis simple.

The maxima of the anomaly (darkest shade of gray) at each frequency arerelatively well-aligned, almost parallel to the 3 km-constant-offsetline 1004. This second electric distance fairly corresponds to thesecond edge of the anomaly—in fact the known anomaly length for thissimulated data calculation is 2 km. Thus the difference between thefirst two electric distances (3−1.2=1.8 km) is a reasonable estimate ofthe length of the anomaly, i.e. the lateral dimension of the anomalyalong the direction of the survey source's movement. Map attributes,such as the gradient, can help determine the most accurate location ofthe maxima.

A third electric distance corresponds to the third dimension of theanomaly (its width in this simple instance). However, it is usually notvery well defined because the anomaly shape may be irregular and itseffect starts to interfere with other potential anomalies and backgroundvariation. Nevertheless, it can be seen from FIG. 10 that there is arange of offsets beyond 3 km where the relative amplitude contours againhave comparable orientation, and line 1005 is an estimate of the slopeat which this occurs.

FIG. 11 shows the relative phase (difference between the phase of theactual data and the phased of the reference) in the Scaled Offset−ScaledFrequency plane for the same receiver. While FIGS. 4A-B showed that thephase plot was superior to the amplitude plots, the relative amplitudeplot of FIG. 10 is as good as the relative phase plot of FIG. 11. Thisfact is important because the amplitude information of actual data isusually much more reliable than the phase.

FIG. 12 shows the contours of the relative amplitude in theOffset−Frequency plane. It is a better display than FIGS. 3A-C, but thefirst electric distance is not easy to pick from the zero-contour. FIG.12 is an example of an embodiment of the present inventive method inwhich relative amplitude is displayed in a Scaled Offset/ScaledFrequency plane, but instead of the scale exponents ½, ½ of thepreferred embodiment, this embodiment uses exponents 0, 1. It canreadily be seen from this that it is advantageous to display data in thepreferred scaled offset/scaled frequency plane(Offset×Frequency^(0.5)/Frequency^(0.5)) of the present invention.

The individual graphs of relative amplitude in the Scaled Offset−ScaledFrequency plane for each receiver can be plotted together on a truegeographic base-map as in FIG. 13. Four receivers 1-4 are plotted attheir true geographic location and the scaled offset−scaled frequencygraphs are displayed along the tow line direction 1311. Positive offsets(i.e., in the direction of the tow) are displayed to port (1312).Negative offsets (i.e., in the opposite direction from the tow) aredisplayed to starboard (1313). The scaled-offset and scaled frequencytrue values are further scaled as may be needed to minimizeover-posting. For instance, one can make sure that the new maximum valueof the scaled-offset is less than the average distance between receiversand that the new maximum value of the scaled frequency is smaller thanhalf the average distance between tow lines.

If the data density is high, for instance at lines crossings, thedisplay software can be such that the user can interactively hide orre-show individual graphs. For example, pushing the HideO button on thetool bar 1316, then clicking on the receiver location and on the desiredline towards the positive (1314) or negative (1315) offsets will hidethe selected graph and temporarily simplify the map presented in thedisplay of FIG. 13. Pushing the ShowO button will show the selectedgraph again. Other ways of co-rendering the results of the presentinventive method will occur to skilled users.

The anomaly in this simulated example is 600 m deep below the sea floorand located immediately east of receiver 4; the anomaly outline isindicated by reference number 1302. Negative offsets (direction 1315,away from the anomaly) show only the zero-contour, i.e. log of relativeamplitude is zero. On the positive (1314) offsets, the intensity of theanomaly increases for receivers closer to the edge of the anomaly. Thisis as would be expected.

Measurements made at a given receiver cannot reflect the presence of aresistive body lying east of the receiver for source positions west ofthe receiver (negative offsets). Thus the negative offset data willreflect background. Positive offset measurements will reveal the anomalyfor offsets sufficiently long. The closer a receiver is to the resistivebody, the shorter the offset required to reveal the onset of the anomalyand the greater the measurement (gray shading) of the anomaly. Thethick, black dotted lines 1303-1306 corresponding to the first electricdistance gets steeper from receiver 1 to 4. As illustrated in FIG. 14,the electric distance is equal to the distance H between the receiver1401 and the closest edge of the anomaly (resistive body) 1402 plus thedepth Z of the anomaly; i.e., electric distance=H+Z. The offset valuecorresponding to each of the lines 1303-1306 can be determined in scaledoffset−scaled frequency space from the slope of the line. The electricdistances calculated from the measured slopes for receivers 1-4 are 3.7,2.7, 1.8 and 1.2 km, respectively. These values can be shown to agreewith values calculated from the known position and depth of theresistive body using the equation electric distance=H+Z.

More receivers and more lines (parallel and crossing lines) could bedisplayed on maps like FIG. 13. Such maps are a very convenient way todisplay and interpret a large amount of data.

The foregoing application is directed to particular embodiments of thepresent invention for the purpose of illustrating it. It will beapparent, however, to one skilled in the art, that many modificationsand variations to the embodiments described herein are possible. Allsuch modifications and variations are intended to be within the scope ofthe present invention, as defined in the appended claims. Personsskilled in the art will recognize that mapping of CSEM data by thepresent inventive method is best displayed with the aid of a computer,i.e. the invention is computer implemented in its preferred embodiments.In such instances, the resulting anomaly maps may be either downloadedor saved to computer memory.

1. A hydrocarbon prospecting method for displaying electromagnetic fielddata from a controlled source electromagnetic survey of a subsurfaceregion to enhance identification of resistive anomalies in thesubsurface region, thereby locating resistive bodies causing theanomalies, comprising: (a) decomposing electromagnetic data from atleast one survey receiver into frequency domain, and selecting amplitudedata corresponding to at least two frequencies; (b) plotting theselected amplitude data with a scaled amplitude plotted on one axis vs.a scaled offset, where offset is source-receiver separation, on theother, with scale factors being chosen to substantially collapse datafrom the two or more frequencies to a single curve; and (c) identifyingone or more resistive anomalies by comparing positive offset data tonegative offset data; and (d) estimating location of a resistive bodyand potential hydrocarbon source that is causing an anomaly fromlocation of the anomaly on the plot relative to location of the surveyreceivers.
 2. The method of claim 1, wherein the scaled offset is offsetmultiplied by the frequency raised to a first selected power(offset×f^(P1)) and the scaled amplitude is amplitude multiplied byfrequency raised to a second selected power (amplitude×f^(P2)).
 3. Ahydrocarbon prospecting method for displaying electromagnetic field datafrom a controlled source electromagnetic survey of a subsurface regionto enhance identification of resistive anomalies in the subsurfaceregion, thereby locating resistive bodies causing the anomalies,comprising: (a) decomposing electromagnetic data from at least onesurvey receiver into frequency domain, and selecting amplitude or phasedata corresponding to at least two frequencies; (b) plotting a quantityrepresenting the selected electromagnetic field data represented on theplot by contours or color scale on a coordinate plane where the axes arecorresponding scaled frequency and scaled offset, where offset issource-receiver separation, with scale factors being chosen to producesubstantially vertical contours, where vertical means parallel tofrequency axis, in offset ranges not impacted by resistive anomalies;(c) identifying one or more resistive anomalies from the plot; and (d)estimating location of a resistive body and potential hydrocarbon sourcethat is causing an anomaly from location of the anomaly on the plotrelative to location of the survey receivers.
 4. The method of claim 3,wherein the one or more resistive anomalies are identified from theshape of the contours or from color differences when comparing positiveoffset data to negative offset data on the plot or when comparingdifferent receiver locations on the plot.
 5. The method of claim 3,wherein the scaled offset is offset multiplied by the frequency raisedto a first selected power (offset×f^(P1)) and the scaled frequency isfrequency raised to a second selected power.
 6. The method of claim 5,wherein the first selected power is ½ and the second selected power is½, and further comprising identifying at least one constant offsetoblique straight line in the plot passing through the point representingzero offset and zero frequency, said straight line being identified asseparating a portion of the plot indicating background and a portion ofthe plot indicating an anomaly or identified as passing through a peakof the anomaly.
 7. The method of claim 6, further comprising calculatingposition and depth of a resistive body from the offset values of theconstant offset oblique lines that were identified, said offset valuesbeing computed from measured slopes of the identified constant offsetoblique lines.
 8. The method of claim 7, wherein the position and depthcalculation uses the following relationship:electric distance=H+Z where Z is depth of the resistive body belowsurvey receiver level, H is straight line distance from a receiver tothe nearest edge or corner of the resistive body, and electric distanceis an offset value of an identified constant oblique line.
 9. The methodof claim 8, wherein the quantity representing the selectedelectromagnetic field data is the logarithm of a relative dataamplitude, being relative to a background reference, and said identifiedconstant oblique line is a line defining a transition from a zerorelative amplitude region to a non-zero relative amplitude region in theplot.
 10. The method of claim 9, further comprising identifying a secondconstant offset oblique straight line in the plot corresponding tomaximum relative amplitude values, and calculating a second electricdistance as equal to the constant offset of the second oblique line, andestimating a dimension of the resistive body as the difference betweenthe two electric distances.
 11. The method of claim 3, wherein thequantity representing the selected electromagnetic field data is ascaled amplitude equal to amplitude×(frequency)^(P3), where P3 is athird selected power.
 12. The method of claim 3, wherein the quantityrepresenting the selected electromagnetic field data is a relative dataamplitude or phase, being relative to a selected reference.
 13. Themethod of claim 12, wherein relative amplitude is amplitude of selectedelectromagnetic field data divided by a selected reference amplitude, orsome function of that amplitude ratio such as the logarithm of theamplitude ratio.
 14. The method of claim 13, wherein the selectedreference amplitude is an amplitude value representative of portions ofthe subsurface region containing no anomalies, i.e. background.
 15. Themethod of claim 12, wherein relative phase is the difference betweenactual phase and an arbitrary reference phase.
 16. The method of claim3, wherein the scaled-offset axis is parallel to a survey sourcemovement line.
 17. The method of claim 3, further comprisingco-rendering in a display maps (plots from step (b)) for a plurality ofreceivers whose locations fall on the scaled-offset axis, and using thismulti-receiver map in step (c).
 18. The method of claim 17, whereinpositive-offset data are plotted in one half (upper half or lower half)of the multi-receiver map, and negative-offset data are plotted in theother half.
 19. The method of claim 18, wherein said co-rendered displayhas capability to temporarily turn off data corresponding to individualreceiver positive or negative offset data to simplify the display andfacilitate resolution of over-posting conflicts.
 20. The method of claim3, wherein the survey is conducted with a plurality of stationaryreceivers and a moving source, said source moving in one or morestraight lines with a plurality of receivers aligned along each sourceline.
 21. The method of claim 3, wherein the quantity representing theselected electromagnetic field data is an electromagnetic field dataattribute.
 22. The method of claim 19, wherein the attribute is gradientof a scaled amplitude, where scaledamplitude=amplitude×(frequency)^(−1.5).
 23. The method of claim 22,further comprising co-rendering on the same plot with the gradient asecond quantity representing the selected electromagnetic field data.24. The method of claim 23, wherein the second quantity representing theselected electromagnetic field data is a scaled amplitude, where scaledamplitude=amplitude×(frequency)^(−1.5).
 25. A method for producinghydrocarbons from a subsurface region, comprising: (a) conducting acontrolled-source electromagnetic survey of the subsurface region; (b)obtaining a map of survey results, said map prepared by stepscomprising: (i) decomposing electromagnetic data from at least onesurvey receiver into frequency domain, and selecting amplitude or phasedata corresponding to at least two frequencies; (ii) plotting a quantityrepresenting the selected electromagnetic field data represented on theplot by contours or color scale on a coordinate plane where the axes arecorresponding scaled frequency and scaled offset, where offset issource-receiver separation, with scale factors being chosen to producesubstantially vertical (parallel to frequency axis) contours in offsetranges not impacted by resistive anomalies; (iii) identifying one ormore resistive anomalies by comparing positive offset data to negativeoffset data or by comparing other different receiver locations on theplot; and (iv) estimating location of a resistive body and potentialhydrocarbon source that is causing an anomaly from location of theanomaly on the plot relative to location of the survey receivers; (c)drilling a well and completing it into the located resistive body; and(d) producing hydrocarbons from the well.